1. Technical Field
The present invention relates to a quartz crystal resonator element, a quartz crystal device including the quartz crystal resonator element, and a method for producing the quartz crystal resonator element. Particularly, the invention relates to a quartz crystal resonator element suitable to increase a space occupied by a thin section effective to promote excitation in production of an inverted mesa-type quartz crystal resonator element or the like, as well as relates to a quartz crystal device including the quartz crystal resonator element, and to a method for producing the quartz crystal resonator element.
2. Related Art
The inverted mesa-type quartz crystal resonator element is adaptable for high-frequency use while securing mechanical strength in a quartz crystal resonator element. In the recent years, there has been an increased demand for miniaturization of the inverted mesa-type quartz crystal resonator element. However, smaller and thinner quartz crystal resonator elements often cause two major problems: a problem due to the resonator element itself and a problem occurring between the resonator element and other constituent components in production of a quartz crystal device. Of the above problems, the resonator element-related problem can be described as follows.
For example, there is a difficulty in securing a space for providing a thin section as a resonating section. Specifically, to form a configuration of an increasingly smaller quartz crystal resonator element, wet etching is often used because of its high mass productivity. However, wet etching of quartz crystal is influenced by crystallographic orientation of quartz crystal. When producing the inverted mesa-type quartz crystal resonator element, the etching rate varies depending on each crystal face appearing on an etched surface. As a result, etching residues form an inclined face (a crystal face) around the thin resonating section.
As a quartz crystal substrate used in the resonator element is smaller, an amount of the residues appearing from a thick section to the thin section is increased, whereby an effective area of the thin resonating section is reduced. This also results in reduction of a size of an excitation electrode. With an extremely small excitation electrode, a stable signal cannot be extracted from the quartz crystal, or sufficient energy trap cannot be expected. Thus, a graph of frequency-temperature characteristics sometimes shows influence of stress or the like from a supporting portion, or active dips.
In order to solve those problems, for example, JP-A-2002-33640 and JP-A-2001-144578 disclose techniques for increasing a space for the thin section in the inverted mesa-type quartz crystal resonator element.
Regarding the inverted mesa-type quartz crystal resonator element provided in JP-A-2002-33640, an unnecessary part of the thick section is cut by dicing after producing the resonator element.
Additionally, in the inverted mesa-type quartz crystal resonator element disclosed in JP-A-2001-144578, a thin section is formed by grinding and the resonator element is individually obtained by dicing.
On the other hand, the problem between a quartz crystal resonator element and other components in a quartz crystal device is as follows. For example, stress generated by mounting a quartz crystal resonator element on a package or a substrate has influence on frequency characteristics. Specifically, when the quartz crystal resonator element is mounted on the package or the substrate using a conductive adhesive or a bump, the resonator element is influenced by stress loaded onto the package or the substrate or stress due to a difference in linear expansion coefficient between quartz crystal and the package or the substrate. In this case, the resonator element causes deviation in its frequency characteristics.
The influence of the problems above becomes greater as quartz crystal resonator elements become smaller and thinner. Thus, studies have been promoted regarding a relationship between frequency characteristic changes and stress (stress sensitivity). As a result of the studies, for example, there is provided a document “The Force Sensitivity of At-cut Quartz Crystals” (J. M. Ratajski, 1966) that describes stress sensitivity of an AT-cut quartz crystal substrate. The document shows that, on the AT-cut quartz crystal substrate, a direction having a lowest stress sensitivity is a direction in which an X axis as a crystallographic axis is rotated by 60° or 120° in a direction from a −X axis to a −Z′ axis about a Y′ axis.
The inverted mesa-type quartz crystal resonator elements disclosed in the above patent documents surely increase the space occupied by the thin section. However, the resonator elements both require machine processing to produce the resonator elements. Accordingly, mass productivity is reduced as compared to a quartz crystal resonator element produced using only wet etching.